text: Three modes allow the addition of allylstannanes to carbonyls: thermal addition, Lewis-acid-promoted addition, and addition involving prior transmetalation. Each of these modes invokes a unique model for stereocontrol, but in all cases, a distinction is made between reagent and substrate control. Substrate-controlled additions typically involve chiral aldehydes or imines and invoke the Felkin-Anh model. When all reagents are achiral, only simple diastereoselectivity (syn versus anti, see above) must be considered. Addition takes place via an S' mechanism involving concerted dissociation of tin and C-C bond formation at the γ position.
  With the allylstannane and aldehyde in high-temperature conditions, addition proceeds through a six-membered, cyclic transition state, with the tin center serving as an organizing element. The configuration of the double bond in the allylstannane controls the sense of diastereoselectivity of the reaction.
  This is not the case in Lewis-acid-promoted reactions, in which either the (Z)- or (E)-stannane affords the syn product predominantly (Type II). The origin of this selectivity has been debated, and depends on the relative energies of a number of acyclic transition states. (E)-Stannanes exhibit higher syn selectivity than the corresponding (Z)-stannanes.
  In the presence of certain Lewis acids, transmetalation may occur before addition. Complex reaction mixtures may result if transmetalation is not complete or if an equilibrium between allylic isomers exists. Tin(IV) chloride and indium(III) chloride have been employed for useful reactions in this mode.